#include <cstdio>
#include <cmath>
#include <iomanip> 

int main()
{
	double a, b, c;
	scanf("%lf%lf%lf", &a, &b, &c);
	double p = 0.5 * (a + b + c);
	double s = sqrt(p * (p - a) * (p - b) * (p - c));
	printf("%.1lf\n", s);
	
	return 0; 
}
